Heavy-duty vehicles are typically equipped with air brakes operated by compressed air that is lead through a system of air brake hoses and valves to the brakes. Connecting multiple trailers on vehicles such as trucks, trailers and semi-trailers to the brake system may be conveniently done using flexible air brake tubing that can be either coiled or straight. However, the conditions of the road and safety concerns place many requirements on the tubing that is exposed to the environment.
Even in normal use, air brake tubing is often subject to environmental and mechanical stresses. Therefore, to ensure safe operation of vehicles, air brake tubing is subject to various government and industry regulations. The quality and performance of air brake tubing is controlled, for example, by SAE Standard J2494 (Push-To-Connect Tube Fittings for Use in the Piping of Vehicular Air Brake), SAE Standard J1402 (Automotive Air Brake Hose and Hose Assemblies), SAE Standard J844 (Nonmetallic Air Brake System Tubing), SAE Standard J2547 (Nonmetallic Air Brake System Tubing), and SAE Standard J1131 (Performance Requirements for SAE J844 Nonmetallic Tubing and Fitting Assemblies Used in Automotive Air Brake Systems), as well as NHSA/DOT FMVSS 106 (49 C.F.R. § 571.106).
Air brake tubing sizes can vary with slightly different requirements for each size. For example, acceptable tubing sizes include outer diameter (O.D.) ⅛ inch, 5/32 inch, 3/16 inch, ¼ inch, 5/16 inch, ⅜ inch, ½ inch, ⅝ inch and ¾ inch, with corresponding nominal inside diameter (I.D.) of 0.156, 0.187, 0.218. 0.281. 0.343, 0.406, 0.531, 0.656 and 0.800 inches. Typically, air brake tubing with an O.D. < 5/16 inch is type A non-reinforced tubing, and above ⅜ inch O.D. is type B reinforced tubing.
In the past air brake hoses have most commonly been manufactured from polyamides such as polyamide 11, polyamide 12 or polyamide 6, copolyesters, or polyamide/copolyester alloys as taught by, for example, U.S. Pat. No. 6,670,004 and U.S. Patent Application Publication No. 2007/0087150 A1. One problem with tubing prepared exclusively from polyamide, for example, PA12, is meeting SAEJ844/DOT FMVSS 571.106 cold temperature impact testing requirements, such as those described in sections 11.3.10 and 11.3.11 of DOT standards, and section 9.10 in SAE standard.
Alternative tubing composition constructions have been explored due to global supply shortages of polyamides. Many tubing manufacturers have turned to the use of copolyesters; however, most commercially available copolyesters require further manipulation in order to meet relevant SAE and DOT requirements. For example, tubing formed from a copolyesters alone, or even a blend of two different copolyesters of different durometers, can come close to failing the SAEJ844 stiffness test. In addition, the all copolyester design suffered from limited customer acceptance compared to the previously proven polyamide design. Prior attempts at resolving these problems resulted in tubing formed from a blend of copolyester and polyamide that improved tubing stiffness, but cold temperature impact performance was inconsistent from lot to lot. Other disadvantages of air brake hoses formed from a polyamide/copolyester blends include poor chemical resistance, and moisture issues.
It would be beneficial to provide a composition for making air brake hoses that meets all of the performance requirements and utilizes materials that are cost effective and widely available.